A sight Lc has been issued but the applicant wants to change the sight lc to be a ussance lc. Is that possible?

the applicant can not change lc from sight to ussance without approval of beneficiary.

Yes it is possible to do the LC change type where 2 changes will be there; you will have to reverse the contra / liability posted at time of sight issuance and post a new contra / liability of the usance LC. Also, the new usnace type linked to a different credit facility, you have to refund the sight credit facility and book from the new usance credit facility.